A typical disc drive includes one or more magnetic discs mounted for rotation on a hub or spindle. A typical disc drive also includes a transducer supported by a hydrodynamic air bearing which flies above each magnetic disc. The transducer and the hydrodynamic air bearing are collectively referred to as a data head. A drive controller is conventionally used for controlling the disc drive based on commands received from a host system. The drive controller controls the disc drive to retrieve information from the magnetic discs and to store information on the magnetic discs.
An electromechanical actuator operates within a negative feedback, closed-loop servo system. The actuator moves the data head radially over the disc surface for track seek operations and holds the transducer directly over a track on the disc surface for track following operations.
Information is typically stored in concentric tracks on the surface of magnetic discs by providing a write signal to the data head to encode flux reversals on the surface of the magnetic disc representing the data to be stored. In retrieving data from the disc, the drive controller controls the electromechanical actuator so that the data head flies above the magnetic disc, sensing the flux reversals on the magnetic disc, and generating a read signal based on those flux reversals. The read signal is typically conditioned and then decoded by the drive controller to recover data represented by flux reversals stored on the magnetic disc, and consequently represented in the read signal provided by the data head.
A typical read back system includes the data head, preconditioning logic (such as preamplification circuitry and filtering circuitry), a data detector and recovery circuit, and error detection and correction circuitry. The read back system can be implemented either as discrete circuitry, or in a drive controller associated with the disc drive.
In disc drives, it is important that the error rate per number of bits recorded (the bit error rate) be maintained at a relatively low level. In a functioning disc drive, bit error rate can be estimated in a number of ways. For example, various data patterns can be continuously written to and read from the discs in the disc drive, and the number of errors encountered in reading the data can simply be counted. During the development of next generation disc drives, however, it is not uncommon for a drive manufacturer to test and evaluate a number of different data heads, or different types of data heads, before choosing a data head for implementation in the next generation disc drive. The development of data heads often precedes the development of read channel circuitry which will eventually be used with those data heads. Therefore, it becomes difficult to estimate the bit error rate of a system employing such data heads, without the read channel circuitry being available.
In the past, others have attempted, without success, to estimate the bit error rate of a system employing such a head (where the read channel circuitry is unavailable) by taking measurements which were thought to be at least of some help in the estimate. Typically, such measurements would include the measurement of the signal recorded on the disc, and noise, to the extent this could be measured. However, such prior measurements have not correlated well to the bit error rate encountered in employing such data heads in a disc drive system.
The present invention is directed to a system that addresses these and other problems, and offers other advantages over the prior art.